YMCA
The national organization is called YMCA of the USA on first reference. It is headquartered in Chicago, Ill. On subsequent references, use YMCA.
Be aware how local YMCA locations are named, such as Melrose Family YMCA, Wang YMCA of China Town, Greenpoint Branch YMCA, John R. Grubb Community YMCA, YMCA of the Prairie, etc.
Note: The 92nd Street Y in New York, N.Y., is not affiliated with YMCA of the USA.